A friend is having a birthday party in Key West on 21 June. What’s the weather like then?
I live in South Florida…and I’d expect the weather to be similar to most of this area. Hot and Humid, with scattered hurricanes.
Where I am, we get afternoon thunderstorms that time of year. I don’t know if they extend to the end of the Keys.
-D/a
Hot.
And yes, the Keys get the daily afternoon thunderstorms that time of year. It’s early in the hurricane season, so I wouldn’t worry too much about that.
Be prepared to stay hydrated. Most northerners don’t realize how quickly they get dehydrated. Although it may feel like it’s hotter than the surface of the sun outside, the ocean breezes really cool you off and you don’t realize you’re leaking liquids at the pace you are. So drink a lot of water (my rule: alternate one glass of water with every booze drink, water, then booze, water, booze).
Also slather the entire body in something like SPF 5000 or so. Do that while naked. If it is done while clothed, the clothing will probably shift by a half an inch. If un-SPFed skin sees the sun for numerous hours, it can result in 2nd degree burning bordering on 3rd degree. A lot of northerners also don’t realize how fricking intense the sun is in the tropics.
Native Southern Californian, and I lived in the desert for 11 years. 
As long as the humidity isn’t as bad as New Orleans, I should be OK.
Um… I have terrible news for you.
Alas!